1. Field of the Invention
This invention relates to an electrical brush for making electrical connection to an object having a predetermined shape and a predetermined orientation relative to the brush, such as a slip ring in a motor or electrical generator, or a stationary contact in a switch. This invention also relates to a method of making such an electrical brush.
2. Description of the Prior Art
Electrical brushes for utilization in electrical applications have long been known in the prior art. Perhaps the earliest modern electrical brush was disclosed by Edison in U.S. Pat. No. 276,233, which resulted in numerous suggested improvements on electrical brushes, as well as related inventions which have otherwise never found significant application.
Thomson, in U.S. Pat. No. 539,454, recognized various advantages of electrical brushes constructed of plural lightly metalized carbon filaments, and in particular the improved brush conductivity, elasticity and reduced mechanical and electrical resistance thereby provided.
More modern development of electrical brushes is evidenced in U.S. Pat. No. 3,668,451 to McNab and U.S. Pat. No. 3,821,024 to Wilkin et al. In the McNab patent is disclosed an electrical brush formed of refractory nonconducting fibers, each of which has deposited thereon a metal film on the surface thereof to carry current. According to McNab, the fibers can be of very small diameter, less than 10,000ths of an inch, and with a relatively thin metallic coating resulting in a considerably more flexible brush having greater current carrying capacity than the brushes known prior to that time. In the Wilken et al patent, an electrical brush is constructed using carbon fibers coated with an underlayer of nickel and an outer layer of silver having an average filament diameter of 7.5 .mu.m coated with metal layers estimated as having thicknesses of on the order of 1 .mu.m. According to Wilkin et al, improved electrical performance is thereby attained due to the fact that the nickel underlayer adheres better to the carbon fiber while making excellent connection to the silver outer layer. In addition to nickel, underlayers of chromium, iron and cobalt are identified as being suitable, while overlayers of gold, copper and alloys of silver and copper are also identified as being suitable overlayers.
Insofar as the prior art methods of making fiber brushes are concerned, these methods were rather straightforward as long as metal fibers or wires having diameters of 100 .mu.m or more were used, namely via the mechanical assembling of bundles of fibers like ordinary bruushes. In that case one may begin with already assembled wire or fiber materials such as grounding cables, spooled wire or fibers, or woven material out of which the weft, for example, is removed, leaving only the warp. With carbon fibers such methods are feasible down to much smaller diameters since carbon fibers are commercially available in tows and at relatively modest cost, including diameters of the individual filaments on the order of 10 .mu.m. With metals, the cost of wire material rises very steeply with decreasing diameters and becomes prohibitive.
A grave disadvantage of mechanical methods of brush making using fibers of small diameters is the difficulty of reliably adjusting the packing density on a small scale, as well as to shape the brush surface to conform to the surface of an object to which the brush is ultimately required to make electrical contact. Shaping of the brush surface is further complicated where an angle of attack other than 90.degree. is required to make contact with the object, for example, a rotor in an electrical motor or generator. Shaping of the brush is not necessary for brush diameters that are sufficiently small. Also, it does not pose much of a problem if the packing density is high, for example, 25% or higher depending on fiber smoothness, since at such packing density the internal friction among the fibers renders the brush relatively stiff. However, at low packing density serious problems are otherwise encountered.
Various methods, as represented by U.S. Pat. No. 3,394,213 to Roberts et al and U.S. Pat. No. 3,277,564 to Webber et al, are disclosed in the prior art for forming microscopic filaments of long length. As taught by Webber et al, a sheathed wire is firstly drawn down through a suitable die to reduce the diameter of the wire within the sheath, whereupon a plurality of the reduced sheath wires are then disposed within a sheath formed of a suitable matrix material which may but need not necessarily comprise the same material as the sheath. The bundle of sheathed wires is then drawn down to define another reduced diameter, which can be successively drawn down to even smaller diameters as may be required for a particular application. Individual filaments of reduced diameter are then obtained from the final bundle by etching away the matrix material. In the Roberts et al disclosure, plural filaments having a diameter of under 15 .mu.m are formed by providing in a housing material a bundle of substantially parallel sheathed elongated drawable elements from which the filaments are to be formed, evacuating the housing, heat forming the evacuated housed bundle, cold drawing the bundle to further reduce the cross-section of the elements therein and then removing the housing and sheathing materials by means of etching.
Another prior art patent of interest is U.S. Pat. No. 3,818,588 to Bates, which discloses an electrical brush constructed by molding an aligned array of metal coated carbon fibers onto a block. According to Bates, the block may be several times the required length and width of a brush, in which case it is then cut into strips corresponding to the desired length of the brush. The coating is then removed for part only of the lengths of the brush to expose the individual carbon fibers at one end but leaving them consolidated for connection to a conductor at the other end, whereupon the strips are finally cut up to form individual brushes.
Although the concept of metal fiber electrical brushes is not of itself new, widespread introduction of metal fiber brushes has been prevented, presumably for several reasons. Firstly, fiber brushes tend to be more expensive than solid, i.e. "monolithic" brushes. Secondly, the monolithic graphite brush was successfully improved to the point that from the technical viewpoint, its losses are easily tolerable for the large majority of common applications, its lifetime is long, and its cost low, albeit the cost of energy lost in the brushes will often exceed their cost. Thirdly, while the braod concept of fiber brushes was known, a theoretical understanding of the interrelationship of brush parameters, such as packing density, fiber diameter, brush pressure and fiber length, as well as experimental testing, was lacking, thereby effectively precluding derivation of optimum brush parameter combinations. Additionally, past failure to achieve superior performance hypothesized for metal brushes may have further discouraged purposeful research, to the extent that metal fiber electrical brushes exhibiting the expected performance have not heretofore been available.
During the past several years, a new interest in the development of improved brushes, whether fiber or monolithic, has arisen due to the development of engineering concepts and planned devices which call for very low "noise" of the brushes, or very high current densities, or high relative speeds, often with only small potential differences driving the currents, demanding much lower losses per ampere conducted than was previously permissible, or any combination of the above conditions. As a result, the prior art brushes cannot meet the envisioned considerably more stringent requirements, necessitating the development of the improved metal fiber electrical brush of the invention.